Using data from the super-sample of the "PISA Suisse" 2003 assessment, this article examines the relationship between the characteristics of education systems (made up of homogeneous or heterogeneous tracks) and their consequences in terms of effectiveness and equity. Our results indicate that it is not so much the official structure of tracks as the ways in which tracking is really organised--and, in particular, the degree of segregation that tracking methods involve--which make it possible to explain inequalities among students. They also show that some education systems are more inegalitarian than others and that the factors leading to inequalities can vary significantly from one Swiss canton to another. Finally, through multilevel analyses, we demonstrate that when the individual and aggregate characteristics of students are taken into account, the type of track a student attends is of only limited significance for explaining inequalities, which suggests that the effects of tracking are in fact linked to the nature of the population educated in the tracks. (Contains 7 tables, 1 figure, and 14 notes.)